The present invention relates to a process for producing hexamethyleneimine by catalytic hydrogenation of .epsilon.-caprolactam.
Hexamethyleneimine is a useful compounds as a physiologically active substance, particularly as an intermediate material for agricultural chemicals.
As processes for producing hexamethyleneimine, there have been heretofore proposed a process comprising reducing .epsilon.-caprolactam with a metal hydride such as LiAlH.sub.4 or HaBH.sub. 4 (for example, Hel.chim.Acta. 32, 544(49). Tetrehedron Letters 1968 (1) 61. ) and a process comprising deammonizing hexamethylenediamine (for example, see, Ber. 96,924(1963), J. Chem. Soc. Japan, Pure Chem. Sec. 82, 1700 (1961) ). However, these known methods are accompanied by various problems such as low yield and high cost of starting materials, and therefore they are not suitable for industrial purposes.
The reaction process for producing a cyclic amine by a catalytic hydrogenation of a cyclic amide tends to cause a side reaction such as a ring cleavage reaction which makes it difficult to obtain the cyclic amine in a good yield. In addition, this process entails a great number of problems which must be solved, such as a remarkable reduction in reaction rate due to the formation of polyamides due to the polymerization of the cyclic amide and a reduction in the activity of the catalyst due to the poison action of the amino acid formed.
It is known that a copper-chromium catalyst [for example, Homer Adkins. J.A.C.S. 56, 2419 (1934) ] and a rhenium catalyst [for example, H.-Smith Broadbent, J.O.C. 24, 1847 (1959) ] are excellent catalysts for the production of an amine by catalytic hydrogenation of an amide. However, when the catalytic hydrogenation is carried out with the use of these catalysts, for example, the copper-chromium catalyst, the reaction must be carrid out at a temperature of 200.degree. to 300.degree. C and at a pressure of 200 to 300 kg/cm.sup.2 G in the presence of a large quantity amounting to 10 to 20% or more of the catalyst. The use of such a large quantity of the catalyst under a high pressure is considered to be necessary because the reaction must be completed as rapidly as possible since the water formed decomposes the amide into an acid which poisons the catalyst. On the other hand, in the case of the use of the latter rhenium catalyst, the reaction is carried out at a temperature of 150.degree. to 250.degree. C and at a high pressure of 200 to 300 kg/cm.sup.2 G, although the catalyst is used in a quantity of 1 to 2%, based on the amount of the material to be reacted.
Also, a method for producing hexamethyleneimine by reduction of .epsilon.-Caprolactam in the presence of a catalyst for hydrogenation has been proposed (U.S. Pat. No. 2181140). In this method, the catalyst used comprises a single member selected from the group consisting of nickel and cobalt of the VIII Group elements and copper of the IB Group and zinc and cadmium of the II Group B elements, respectively, in the periodic table, or a combination of these elements. However, the reduction reaction is also carried out at a pressure as high as 200 atmospheres. Although the specification of this patent does not disclose the conversion of the .epsilon.-caprolactam and the yield of the hexamethyleneimine in detail, it may not ne considered that this method can overcome the afore-mentioned difficulties such as the activity reduction due to catalyst poisoning.
In fact, when we carried out the catalytic hydrogenation of the .epsilon.-caprolactam with the use of the known catalyst mentioned above, it was expectedly found that the catalyst was of low activity and of no use for industrial purposes. Therefore, there is a need for a novel catalyst for this technique.